This invention relates to drive circuits for metal oxide semiconductor field effect transistors (MOSFET) and, more particularly, to such circuits in which the MOSFET acts as a current source.
Remote power controllers are used to control loads in power distribution systems, such as those found on aircraft. In some applications, it is desirable to limit the current provided by the power controller to a specific level in the case of an overload. A remote power controller which uses a MOSFET power circuit for current limiting is disclosed in my commonly assigned, copending patent application Ser. No. 07/290,661, filed Dec. 27, 1988, now U.S. Pat. No. 4,914,542, and entitled "Current Limited Remote Power Controller".
When a MOSFET is used as a current source, if the voltage across the MOSFET changes rapidly, then the current will also change because of the gate to source capacitance within the MOSFET. In a typical MOSFET current source circuit, the gate of the MOSFET appears as a capacitor to the drive circuit. Internal capacitance exists between the gate and both the drain and the source. When the source to drain voltage of the circuit changes, the gate to drain voltage also changes because of the internal capacitor divider in the MOSFET. This changes the level of current through the MOSFET. This abnormal current condition will remain for a time determined by the time constant of the drive circuit resistance and the gate capacitance.
A number of previous solutions to this undesirable current variation exists. Placing a large capacitor from the gate to the drain or placing an inductor in series with the drain or both, will reduce this effect. However, these solutions also slow down the switching speed of the MOSFET. Another technique for reducing this effect is to keep the output resistance of the drive circuit low. However, this results in a relatively complex drive circuit.
It is therefore desirable to devise a MOSFET drive circuit which is capable of reducing the change in output current in response to a voltage change across the MOSFET without reducing the switching speed of the MOSFET and without using complex, low impedance drive circuitry.